The production of integrated circuits typically includes two testing stages: DC testing and AC testing. The DC testing is typically performed after the fabrication of the circuits on a semiconductor wafer using a "bed of nails" probe. The measurements taken during DC testing typically include power supply current, output sink and source current, input and output logic high/low voltages, along with other such tests. DC testing can be performed quickly for each circuit on the wafer, therefore, it is relatively inexpensive.
After DC testing, the wafers are assembled into individual integrated circuits. Very often, assembly is performed at a remote location. After assembly, AC testing is performed on the packaged units. Importantly, the speed at which an integrated circuit may operate is determined. Because of processing variations, the speed of the chips may vary from wafer-to-wafer, and even between individual circuits on a single wafer.
In many cases, parts are "screened" to classify individual integrated circuits at various speeds. For example, a part may be produced in three versions: 16 MHz, 20 MHz and 25 MHz. It would be advantageous to predict the speed of an individual integrated circuit prior to assembly; however, an expensive AC test is required to perform such measurements. Because of the cost involved in performing AC testing after processing stage, screening is generally performed after assembly.
Therefore, a need has arisen in the industry to provide a method and apparatus for predicting AC delays after processing without the need for expensive test equipment.